Meaning of Umbreon (when they say that): any word with 7 or more letters.
Meaning of umbr, reon (when they say that): any word with 4, 5, or 6 letters.
Meaning of mbr, umb, eon, mbre (when they say that): any word with 3 letters.
Meaning of on, um, br (when they say that): any word with 2 or less letters.

How can you tell
You can tell by how they say that
Like if an Umbreon said
Umbr reon
In a happy tone it will mean lets play